Exhaust systems are widely known and used with combustion engines. Typically, an exhaust system includes exhaust tubes or pipes that convey hot exhaust gases from the engine to other exhaust system components, such as mufflers, resonators, etc. Mufflers and resonators include acoustic chambers that cancel out sound waves carried by the exhaust gases. Although effective, these components are often relatively large in size and provide limited nose attenuation.
One solution for reducing low frequency noise is to use an adaptive/passive valve assembly. The passive valve assembly includes a flapper valve body or vane that is positioned within an exhaust pipe where the vane is pivotable between an open position and a closed position. The passive valve is spring biased toward the closed position, and when exhaust gas pressure is sufficient to overcome this spring bias, the vane is pivoted toward the open position. When the exhaust gas pressure falls, the spring returns the vane to the closed position. As the vane moves back and forth between the open and closed positions, spring coils have relative torsional displacement relative to each other.
With the use of the spring, it is difficult to return the vane to a consistent closed position within the exhaust pipe. Further, while effective at attenuating low frequency noise, the introduction of the passive valve into the exhaust system presents additional noise challenges. For example, the spring of the valve assembly can generate an undesirable chattering noise, especially when in the closed or idling position. In this position, the valve can flutter slightly back and forth due to small fluctuations in exhaust gas flow creating an undesirable torsional flutter condition.